1. Field of the Invention
The present invention relates to an RFID (Radio Frequency Identification) tag that performs information exchange with external equipment in a noncontact manner. Incidentally, among those skilled in the art related to the technical field of the present application, the “RFID tag” used in the specification of the present application may sometimes be called an “inlay for RFID tag” by regarding the “RFID tag” as an internal component member (inlay) for “RFID tag.” Or alternatively, in some cases, this “RFID tag” may be referred to as “a radio IC tag.” Also, a noncontact type IC card is included in this “RFID tag.”
2. Description of the Related Art
In recent years, there have been proposed various RFID tags that perform information exchange with external equipment represented by a reader/writer in a noncontact manner by use of radio waves (refer to U.S. Pat. No. 6,239,703B1, for example). As one kind of this RFID tag, there has been proposed an RFID tag in which an antenna pattern for radio communication and an IC chip are mounted on a base sheet made of plastics or paper. A conceived mode of using an RFID tag of this type is such that the RFID tag is stuck to an article and the like and performs the identification of the article by exchanging information on the article with external equipment.
In such an RFID tag, a circuit chip and an antenna pattern are electrically connected together by pinching a micro electric conductor called a bump between the circuit chip and an end portion of the antenna pattern. The circuit chip is fixed to the above-described base sheet and on this occasion, the area between the circuit chip and the end portion of the antenna pattern except the part occupied by the bump is filled with an adhesive. As a result, it follows that a micro capacitor is formed, with the circuit chip and the end portion of the antenna pattern serving as electrodes. The capacity of such a capacitor that is unnecessary in terms of design is called a parasitic capacity, and a parasitic capacity in an RFID tag has an adverse effect as described below.
There is a type of RFID tag that obtains operating power of an internal circuit chip from external equipment, and in this type of RFID tag the power from external equipment is supplied to the circuit chip via an antenna pattern. When at this time, a parasitic capacity exists between the mutually electrically connected circuit chip and end portion of the antenna pattern, the input of operating power from the antenna pattern to the circuit chip is interfered with. As a result, there is a possibility that troubles such as the deterioration of the communication distance of the RFID tag due to insufficient operating power might be caused.
Therefore, there have been proposed techniques which involve providing, on the antenna pattern side, an adjustment pattern capable of changing an impedance by working such as etching, and adjusting an impedance including the above-described parasitic capacity between the antenna pattern and the circuit chip by working this adjustment pattern (refer to U.S. Pat. No. 6,535,175B2, for example).
However, such adjustment work has the problem that productivity is lowered thereby.
Hence, measures against the above-described parasitic capacity in an RFID tag often involve predicting the parasitic capacity and providing, on the antenna pattern side, a pattern which generates an inductance which cancels out the parasitic capacity by a circuital resonance.
The above-described prediction of a parasitic capacity in an RFID tag is performed on the basis of the area of a portion in which the circuit chip and the end portion of the antenna pattern overlap each other, with the above-described bump pinched between the two, (an overlapping portion).
However, there are variations in the positional relationship between the above-described circuit chip and the antenna pattern during the manufacture of an RFID tag, and it is difficult to predict a parasitic capacity in a stable manner. Furthermore, even when a predicted parasitic capacity is determined beyond the limits of reason and an inductance is found on the basis of the parasitic capacity, in a case where an actual parasitic capacity should differ from the predicted parasitic capacity, it becomes impossible to ensure matching between the actual parasitic capacity and the impedance and the above-described resonance does not occur any more. As a result, the parasitic capacity is not canceled and it follows that the above-described problem of deterioration of the communication distance of the RFID tag or the like occurs.